Self levelling floorings contain special dispersants or superplasticizers that are essential to impart perfect fluidity and self-healing properties without excessively increasing the need of water.
Traditional dispersants, like casein and succinoglycan, provide self-levelling characteristics and also prevents the sag of fine aggregates.
Because of their draw-backs (evil smell, volatility in pricing for casein and high cost for succinoglycan) they are now largely replaced by synthetic polymeric dispersants providing excellent flowability (superplasticizers).
Another typical example of flowable hydraulic compositions is flowable concrete, or self-compacting, or self-consolidating concrete (SCC).
Flowable concrete is a non segregating concrete that can be spread into place, fill formwork and encapsulate reinforcements without any mechanical vibration.
The development of flowable concrete closely followed the advent and improve of superplasticizers technology, although the term SCC was coined only in the end of the 1990's, as discussed in “Self-Compacting Concrete: what is new?”, by M. Collepardi (in Proc. 7th CANMET/ACI Int. Conf. on Superplasticizers and Other Chemical Admixtures in Concrete, Berlin, Germany, 20-24 October, p 1-16, 2003).
Superplasticizers provide flowability but do not impart resistance to segregation and bleeding.
Rheology modifying admixtures (RMA) have been therefore developed to enhance the cohesion and stability of cement based systems.
RMA are usually natural polymers or derivatives thereof that enhance the water retention capability, the yield value and plastic viscosity of hydraulic compositions and control bleeding and segregation, without adversely affecting the levelling properties.
Welan gum, diutan gum and cellulose ether derivatives, such as methyl cellulose and hydroxypropylmethyl cellulose are by far the most used RMA for flowable hydraulic compositions.
Welan gum (CAS No. 96949-22-3) is an anionic polysaccharide with excellent thermal stability and retention of viscosity at elevated temperatures.
The welan molecule consists of tetrasaccharide repeating units, each carrying a single sugar branch of either L-mannose or L-rhamnose.
Diutan gum (CAS No. 125005-87-0) is a natural high molecular weight gum produced by carefully controlled aerobic fermentation. The repeating unit is comprised of a six sugar unit. The backbone is made of D-glucose, D-glucuronic acid, D-glucose and L-rhamnose and the side chain of two L-rhamnose.
Diutan gum possesses the highest dose efficiency among the known RMA.
Welan gum and diutan gum are more pseudoplastic than cellulose ethers and effectively prevent particles sag and bleeding at lower concentrations.
For SCC, a dosage of 0.04% by mass of water of diutan is reported to be a very low dosage in “Performance of Self-Consolidating Concrete made with Diutan Gum”, by Khayat, K. H. et al., in American Concrete Institute SP-239, Proc. 8th CANMET/ACI Int. Conf. on Superplasticizers and Other Chemical Admixtures in Concrete, 2006, p. 545-566.
In self-leveling flooring, where coarse aggregates are absent, a lower minimum dosage of diutan gum is recommended in the common practice.
Notwithstanding their low minimum dosages, the major draw-back of welan and diutan gum is their high cost; they are produced in fermentation processes yielding a broth of low biopolymer concentration and are among the most expensive admixtures currently in use (a relative price indication for various additives of natural origin for building material can be found in “Application of biopolymers and others biotechnology products in building materials”, by Plank J., in Appl. Microbiol. Biotechnol. (2004), 66, 1-9).